Method of producing food grade hydrated lime

ABSTRACT

A method is shown for producing food grade hydrated lime. A source of raw, hydrated lime is first passed through a classification step which divides the raw hydrated lime into a first fine stream and a first coarse stream. The first coarse stream is then passed to a grinder which produces a ground coarse product. The first fine stream is separated out from the first coarse stream and, without combining the first fine stream with the first coarse stream or with the ground coarse product, is removed to produce a very fine sized product which meets CODEX chemical specifications.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a method for producing food gradehydrated lime, Ca(OH)₂, which meets the standards as required by CODEXwithout requiring extensive changes in existing equipment or processsteps.

B. Description of the Prior Art

Lime has a variety of uses. It is commonly used in treating waste waterand sewage. It is used in agriculture to neutralize acidic soils and toprovide nutrients for sustaining plant life. Lime is also usedextensively in construction for the stabilization of soils and as acomponent in a variety of building materials. Lime is also used in avariety of “food grade” products intended for human consumption.

In this description, the term “lime” will be understood to mean bothquicklime (calcium oxide) and hydrated lime (calcium hydroxide).Quicklime is produced by heating limestone (calcium carbonate) in a kilnat extreme temperatures to “calcine” the material and thereby drive offcarbon dioxide. Quicklime is usually in the form of lumps or pebbles. Inorder to further process lime and improve the ease with which it ishandled, dry lime is often mixed with water to form a slurry. In thecase of quicklime, the water reacts with the quicklime in an exothermicreaction to form hydrated lime. This is often referred to as slaking.During the slaking of quicklime, large amounts of heat are given offwhich can significantly raise the temperature of the slurry. Water canthen be driven off to produce dry, hydrated lime which is usually apowder.

Food grade hydrated lime is a specific material that is sold to the foodprocessing industry in the United States. The specifications for foodgrade hydrated lime are defined by CODEX. The CODEX AlimentariusCommission was created in 1962 by two U.N. organizations, the Food andAgricultural Organization (FAO) and the World Health Organization (WHO).CODEX is the major international mechanism for encouraging fairinternational trade in food while promoting the health and economicinterests of consumers. Within the United States, CODEX activities arecoordinated by officials from the U.S. Department of Agriculture, theU.S. Food and Drug Administration and the U.S. Environmental ProtectionAgency. In the United States, there are a number of companies thatproduce “normal”, i.e., industrial grade, hydrated lime for industrialuse. However, there are only two companies known to Applicant at thepresent time that produce “food grade” hydrated lime. This is due, atleast in part, to the exacting chemical specifications required byCODEX. Many hydrated limes that are suitable for general industrial use,fail to meet the CODEX standards since they exceed, for example, thelimits for trace metals found in the compositions.

As mentioned above, hydrated lime is produced by first heating limestonein a kiln (calciner) to remove carbon dioxide and form quicklime. Toimprove the handling characteristics of the quicklime, a controlledamount of water is then added to form calcium hydroxide, commonlyreferred to as hydrated lime. To achieve the CODEX chemicalspecifications requires the use of the purest limestone as calciner feedand normally the use of natural gas as a fuel for the kiln. The use ofnatural gas, even though it is more expensive, is preferred over solidfuel such as coal or coke, in that solid fuel tends to contaminate thequicklime and thus the hydrated lime made from the quicklime withcombustion ash.

A need exists for a method for producing food grade hydrated lime whichmeets CODEX specifications without drastic changes in the equipment orprocess parameters presently employed in calcining limestone to producequicklime, or in slaking the quicklime to produce hydrated lime.

A need exists for a method for producing food grade hydrated lime whichwould allow the use of normal limestone as feed to the calciner, withoutrequiring the purest of limestone as feed.

A need also exists for such a process which would allow the use of solidfuel sources in the calciner, rather than requiring the use of moreexpensive natural gas as a fuel source.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forproducing food grade hydrated lime without requiring unusually purelimestone as calciner feed.

It is also an object of the invention to provide a method for producingfood grade hydrated lime which allows the use of typical solid fuels tofuel the calciner, rather than requiring the use of more expensivenatural gas as a calciner fuel.

It is also an object of the present invention to provide a process formanufacturing food grade hydrated lime which separates but retains thelarger coarse particles of hydrated lime, as well as the finer, screenedparticles, so that the larger sized particles can be processed as normalhydrated lime with no waste being introduced into the process.

The present invention concerns an alternative method of producing foodgrade hydrated lime without the stringent control of limestone feed andcalciner fuel. It has been discovered that some “normal” limestonecalciner feed has its impurities distributed over different particlesizes. Additionally, the combustion ash deposited on the quicklime (CaO)also varies in particle size. During the hydration reaction thatconverts calcium oxide to calcium hydroxide, these impurities can beseparated and removed from the calcium hydroxide particles. In this way,a food grade hydrated lime can be produced from quicklime (CaO) thatpreviously did not meet the CODEX chemical specifications.

In the method of the invention, a source of raw, hydrated lime is firstpassed through a classification step which divides the raw hydrated limeinto a first fine stream and a first coarse stream. The first coarsestream is then either recycled in the process or passed to a grinderwhich produces a ground coarse product. The first fine stream isseparated out, without combining the first fine stream with the firstcoarse stream or with the ground coarse product, and is removed toproduce a very fine sized product which meets CODEX chemicalspecifications.

While the method of the invention can be practiced in a one stepseparation process, the method is preferably practiced in a two stepseparation process. The source of raw, hydrated lime is first passedthrough a classification step as before to produce a first coarse streamhaving particle sizes in the range from about +10 to 50 mesh or larger.The first coarse stream is passed to a grinder which produces a groundcoarse product. A first fine stream which is produced in the primaryclassification step is subjected to a secondary classification step andvery fine product is separated out, without combining the very finestream with the first coarse stream or with the ground coarse product,to produce a very fine sized product having a particle size of about−325 mesh or finer which meets CODEX chemical specifications.

Additional objects, features and advantages will be apparent in thewritten description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified, schematic of a prior art process for producingindustrial grade hydrated lime; and

FIG. 2 is a schematic view, similar to FIG. 1, showing the method stepsof the method of the present invention which is used to produce foodgrade hydrated lime.

FIG. 3 is a schematic view, similar to FIG. 2 but showing an alternativescreening method of the invention in which a slurry is used to separateout coarse particles.

FIG. 4 is a schematic view, similar to FIG. 3 showing another method ofusing a slurry to screen and separate out coarse particles.

DETAILED DESCRIPTION OF THE INVENTION

In the discussion which follows, the term “quicklime” will be taken tomean calcium oxide and should not be confused with limestone (calciumcarbonate). As briefly outlined in Applicant's background discussion,quicklime is manufactured from limestone by heating to remove carbondioxide. Quicklime can be converted to Ca(OH)₂ by a slaking processwhere water and CaO are mixed under agitation and temperature to produceCa(OH)₂, known in the industry as slaked lime or lime hydrate.

FIG. 1 is a simplified, schematic diagram which illustrates the priorart process for producing industrial grade hydrated lime. Raw limestoneis first fed to a calciner 11 which is typically a horizontal orvertical kiln. The kiln is fired by burners which typically utilizepulverized coal as a fuel and are capable of reaching calciningtemperatures in excess of 1600° F. The intense heat causes a chemicalreaction as follows:CaCO₃+heat=CaO (quicklime)+CO₂

The quicklime produced in the calciner 11 is then slaked by mixing withan aqueous slaking medium in hydrator 13. This results in an exothermicreaction generating heat and calcium hydroxide:CaO+H₂O=Ca(OH)₂+heat+steam

The size and quality of slaked lime particles in the resulting slurryare dependent on a number of variables. These include the reactivity,particle size and gradation of the quicklime used. Other variablesinclude the amount of water used, the quality of the water, and theamount and type of water impurities. Further, the temperature of thewater and the amount of agitation can affect slaked lime quality andparticle size.

The excess water not converted to calcium hydroxide is heated to steamand the steam is volatized from the solid calcium hydroxide particles.The solid calcium hydroxide leaving the reactor is composed ofindividual calcium hydroxide particles, agglomerated calcium hydroxideparticles, individual impurity particles, and impurities associated withthe individual and agglomerated calcium hydroxide particles. Thesematerials are represented as “dry agglomerate” in the step 15 shown inFIG. 1 and comprise a broad distribution of calcium hydroxide particles.

This broad distribution of particles is then normally screened or airclassified in a step 17 which divides the raw hydrated lime into a firstfine stream 19 and a first coarse stream 21. Because a large percentageof the calcium hydroxide is present in the coarse fraction of stream 21,this fraction is normally ground in a dry grinding step 23 and thenreturned to the fine calcium hydroxide particles in a stream 25, theparticles being mixed in a step labeled as 27 in FIG. 1. During thisprocess all the impurities present in the quicklime, calcium oxide feedare also present in the final calcium hydroxide product which passes outstream 29.

Applicants have discovered that for most quicklime/hydrated limeproductions, the impurities that are important for CODEX specificationsare uniformly distributed in all particle sizes of the calcium hydroxidewhich is formed according to the above described prior art process.However, Applicants have also discovered that certain sources of rawfeed materials (calcium carbonate) allow the production of food gradehydrated lime which meets the CODEX impurity specifications byincorporating an additional process step which removes the large coarserparticles that are formed during the hydration reaction.

The specific process is to take the “raw” calcium hydroxide from thereactor and then screen or air classify the material to remove thecoarse +10 to 100, most preferably +10 to 50, mesh particles. The finefraction can then be further screened or air classified to −325 mesh toachieve the necessary purity. While the preferred method of theinvention is a two step separation process to achieve higher yields offiner product, it is within the scope of the invention to have a single(one size) separation to meet the CODEX specifications. The criticalpoint in the method of the invention is to insure that the coarsefraction of calcium hydroxide is not processed as in the prior art, bygrinding and adding it to the fine food grade calcium hydroxide.

The improved method for producing food grade hydrated lime isillustrated in FIG. 2 of the drawings. Raw limestone is calcined in astep 31, as before. The resulting quicklime is hydrated in a step 33with the dry agglomerate of step 35 being fed to an air classifier in astep 37. The separated out coarse particles in stream 39 pass to a drygrinder 41 where they are reduced in size and fed through stream 43 tomixing point 45.

In the preferred method of FIG. 2, the first fine stream of particles 47are passed to a secondary separation step in air classifier 49. Aresulting relatively coarse stream 51 can be returned to the mixingpoint 45 to be combined with the product in stream 43, thereby producingold product in outlet stream 53. In the improvement of the invention,the very fine particles (−325 mesh) in stream 55 remain separated fromthe second coarse stream 51 and are removed as product through outletstream 57 without combining the first coarse stream 39 or with theground coarse product 43. The product removed through outlet stream 57is a very fine sized product which meets CODEX chemical specifications.

The invention is thus intended to encompass a two step separationprocess in which the coarse calcium hydroxide particles from the firstseparation, which contain a larger portion of the impurities, can beground and added to the coarse fraction of the second size separation(shown being combined in the mixer 45 in FIG. 2). This product can meetthe specification for “normal” hydrated lime with no waste and 100% ofthe quicklime/hydrated lime being utilized in the process. The dottedline 59 in FIG. 2 is intended to represent the option of passing aportion of the first fine stream to old product, rather than furtherprocessing it to produce food grade product, at the option of theoperator.

The following Table I shows the results of seven differentquicklime/hydrated lime systems and how the impurities are affected byparticle size separation. Note that no sample achieved CODEX food gradespecifications with the then currently available hydrated lime. Onlysample #7 achieved CODEX food grade specification with impuritiesremoved in a coarse fraction.

TABLE I Sample # 1 Sample # 2 Sample # 3 Sample # 4 Sample # 5 Sample #6 Sample # 7 −100 −100 −100 −100 −100 −100 −100 FCC specifications HLmesh HL mesh HL mesh HL mesh HL mesh HL mesh HL mesh Calcium hyd. 95.0%min. 96.5 97 96.1 95.7 96.8 95.1 95.5 95.7 93.4 95.2 96.1 96.3 97.1 97.2Acid insoluble 0.50% max 2 0.4 0.3 0.1 0.2 0.1 1.8 1.5 0.3 0.2 1.9 0.71.0 0.2 Magnesium  4.8% max 0.5 0.2 0.35 0.3 0.2 0.3 1.0 0.9 1.7 1.9 1.31.0 0.3 0.2 Arsenic  3 ppm max 0.9-20.0 0.02 0.1 4.0 3.0 1.3 0.6Fluoride 50 ppm max 45 64 65 80 100 56 36 Lead 10 ppm max 0.63 0.8 0.50.7 0.8 0.37 0.44 Heavy metals 30 ppm max 60 <30 <30 <30 <30 60 <30

The invention has been described with respect to an air classifierseparation step, followed by dry grinding of coarse product in onepreferred embodiment of the invention. Thus skilled in the art will alsoappreciate that the separation of coarse from fine particles could alsobe accomplished by using a slurry to separate out the coarse particles.In FIG. 3, quicklime from the calciner 69 passes through a hydrationstep 71 as in the previously described method. In this case, however,the raw hydrated lime is slurried by the addition of an aqueous mediumin a step 73 and the slurry is then screened to separate out the +100 to+325 mesh coarse particles in a stream 75 with the fine particlespassing out in a separate stream 77.

In FIG. 4, quicklime from the calciner 79 is slaked in a step 81 toproduce calcium hydroxide. The slaked calcium hydroxide is then screen,as before, to separate out the +100 to +325 mesh coarse particles in astream 83 with the fine particles in the slurry passing out the separatestream 85.

An invention has been provided with several advantages. The method ofthe invention allows food grade hydrated lime to be produced with littlechange in existing process equipment or processing steps. The method issimple in design and economical to implement. In addition to producing anew food grade product stream, the process can utilize the separatedcoarser hydrate particles by combining these particles with an oldproduct stream to produce an industrial grade hydrated lime.

While the invention has been shown in only two of its forms, it is notthus limited but is susceptible to various changes and modificationswithout departing from the spirit thereof.

1. A method for producing high purity, food grade hydrated lime, themethod comprising the steps of: providing a source of raw hydrated lime;passing the raw hydrated lime through a first classification step whichdivides the raw hydrated lime into a first fine stream and a firstcoarse stream having particle sizes in the range from about +10 to 50mesh; passing the first coarse stream to a grinder which produces aground coarse product; without combining the first fine stream with thefirst coarse stream or with the ground coarse product, subjecting thefirst fine stream to a further classification step to produce a veryfine product stream and removing the very fine stream to produce a veryfine sized product having a particle size of about −325 mesh or belowwhich meets CODEX chemical specifications.
 2. The method of claim 1,wherein the classification step is accomplished by means of an airclassifier.
 3. The method of claim 2, wherein the raw, hydrated limefeed is prepared by slaking quicklime with water.
 4. The method of claim3, wherein the further classification step to which the first finestream is subject also produces a second coarse stream in addition tothe very fine product stream, and wherein the first coarse stream,containing a larger portion of impurities, is added to the second coarsestream to form a less pure product in addition to the very fine sizedproduct which is produced.